Babysitting conflict and property left?

Last year I babysat a little girl. Her family didn't have alot of money so I was pretty easygoing with payments. They left the playpen over here for naptime and a few other children used it too. So one week they didn't have all of the money so I just said 'thats ok, I use your playpen so we can just call it even for this week'. Not long after I was forced to stop watching the children. The mother sent me an email saying they wanted the playpen back. I was a little mad about that but didn't really argue. Now a whole YEAR later and this thing is still sitting in my house. Can I finally sell it?